Add a getter interceptor to the new interceptor scheme. Also cleanup the "this" that turns into a r…

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(revision 14875)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -711,6 +711,7 @@
   }
 
   bool isInterceptorClass(Element element) {
+    if (element == null) return false;
     return element == jsStringClass;
   }
 
@@ -727,17 +728,21 @@
     return false;
   }
 
+  void initializeInterceptorElements() {
+    objectInterceptorClass =
+        compiler.findInterceptor(const SourceString('ObjectInterceptor'));
+    getInterceptorMethod =
+        compiler.findInterceptor(const SourceString('getInterceptor'));
+  }
 
+
   void registerInstantiatedClass(ClassElement cls, Enqueuer enqueuer) {
     ClassElement result = null;
     if (cls == compiler.stringClass) {
       if (jsStringClass == null) {
         jsStringClass =
             compiler.findInterceptor(const SourceString('JSString'));
-        objectInterceptorClass =
-            compiler.findInterceptor(const SourceString('ObjectInterceptor'));
-        getInterceptorMethod =
-            compiler.findInterceptor(const SourceString('getInterceptor'));
+        initializeInterceptorElements();
       }
       result = jsStringClass;
     }